1. Field of the Invention
The present invention relates to image processing and, more particularly, to systems for editing digital images.
2. Background of the Technology
A large number of applications require combining one video image with another video image so as to produce a composite video image. One well-known technique for producing composite video images is commonly referred to as "chroma-key". The chroma-key technique is so named because it uses the chroma portion of a television signal as a "key" to control the formation of a composite image. A chroma-key device is essentially a video multiplexer which selects a video input signal by feeding the chroma signal from one of the two video inputs to decision logic.
As an example application, television stations typically use chroma-key for weather reports. During the weather report, a television camera is directed at a weather reporter standing in front of a vertical sheet, called a matte, which has a predetermined blue hue. The image of the weather reporter and matte, of foreground source, is provided to a chroma-key device, which is programmed to not select video signals that represent portions of the image having the predetermined blue hue. Thus, the chroma-key device effectively separates the image of the reporter from the image of the matte.
At the same time that the previously described operation takes place, a video image of a weather map or satellite view of the earth, either of which may be superimposed with city names, high/low temperatures, and the like, is provided to the chroma-key device as a background source. The decision logic of the chroma key device selects the background source as video output wherever a blue hue is detected in the foreground source. The background source is also displayed on a studio monitor that can be viewed by the reporter. The reporter can then point to positions on the matte that correspond to geographic locations on the background source and the viewer of the television program sees a composite image of a reporter and a weather map.
Such a composite image is the desired output of a chroma-key device.
However, as was noted in U.S. Pat. No. 4,811,084 to Belmares-Sarabia, et al., a major disadvantage of a chroma-key system is that false keys can be produced. For example, weather reporters wearing blue or striped suits, and even blue eyes, may cause a chroma-key system to produce an incorrectly spliced composite. Also, a chroma-key device is used too large a distance between the reporter and the matte causing reflections resulting from false keying, hence restraining the movements of the reporter.
To overcome the problems inherent in chroma-keying, Belmares-Sarabia, et al., discusses a device for video color detection that does not depend on a single color for keying. For example, such a device is also said to discriminate among similar hues by limiting the bandwidth of the hues and/or hue and saturation combinations that can be recognized by the device.
The device disclosed in Belmares-Sarabia, et al., uses an analog process to multiplex television signals. Analog processes, however, are not as versatile in combining images as are digital processes, which can be programmed to apply sophisticated image processing algorithms to a digitized image so as to alter or edit an image. Thus, it would be an advancement in the technology to provide a digital image editing system which can strip a digital image of an object from a background and combine the digital object with a different digital background.
It is often the case that an object is imaged under one lighting condition and is then overlaid on a background that was imaged under another lighting condition. Consequently, the composite image may look artificial. Thus, it would be a further advantage if the editing system could establish the same lighting conditions, or "gamma", for the entire composite image. For example, it may be desirable to have an object that was imaged under fluorescent light inserted into a background that was imaged under full daylight and have the composite image maintain the same lighting condition. The lighting condition of the composite image could even be a third condition such as moonlight.
Accordingly, a need exists to provide a digital image editing system which can separate the digital image of an object from a background against which the object was imaged. It is a further object of the present invention to provide a digital image editing system which can overlay the digital image of an object onto a predetermined background, and then match the gammas of the object and background, and to provide a digital image editing system that is easy to implement and cost-effective to use.